Helpful Car Repair Tips

How Tires Work

Posted by Brian Brown on March 19, 2010 at 3:46 PM

How Tires Work

by Karim Nice

 What All the Numbers Mean

Each section of small print on a tire's sidewall means something:

Tire Type

The P designates that the tire is a passenger vehicle tire. Some other designations are LT for light truck, and T for temporary, or spare tires.

Tire Width

The 235 is the width of the tire in millimeters (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tire when it is on its intended rim size.

Aspect Ratio

This number tells you the height of the tire, from the bead to the top of the tread. This is described as a percentage of the tire width. In our example, the aspect ratio is 75, so the tire's height is 75 percent of its width, or 176.25 mm ( .75 x 235 = 176.25 mm, or 6.94 in). The smaller the aspect ratio, the wider the tire in relation to its height.

Two tires with different aspect ratios but the same overall diameter

High performance tires usually have a lower aspect ratio than other tires. This is because tires with a lower aspect ratio provide better lateral stability. When a car goes around a turn lateral forces are generated and the tire must resist these forces. Tires with a lower profile have shorter, stiffer sidewalls so they resist cornering forces better.

Tire Construction

The R designates that the tire was made using radial construction. This is the most common type of tire construction. Older tires were made using diagonal bias (D) or bias belted (B) construction. A separate note indicates how many plies make up the sidewall of the tire and the tread.

Rim Diameter

This number specifies, in inches, the wheel rim diameter the tire is designed for.

Uniform Tire Quality Grading

Passenger car tires also have a grade on them as part of the uniform tire quality grading (UTQG) system. You can check the UTQG rating for your tires on this page maintained by the U.S. National Highway Traffic Safety Administration (NHTSA). Your tire's UTQG rating tells you three things:

• Tread Wear: This number comes from testing the tire in controlled conditions on a government test track. The higher the number, the longer you can expect the tread to last. Since no one will drive his or her car on exactly the same surfaces and at the same speeds as the government test track, the number is not an accurate indicator of how long your tread will actually last. It's a good relative measure, however: You can expect a tire with a larger number to last longer than one with a smaller number.

• Traction: Tire traction is rated AA, A, B or C, with AA at the top of the scale. This rating is based on the tire's ability to stop a car on wet concrete and asphalt. It does not indicate the tire's cornering ability. According to this NHTSA page, the Firestone Wilderness AT and Radial ATX II tires that have been in the news have a traction rating of B.

• Temperature: The tire temperature ratings are A, B or C. The rating is a measure of how well the tire dissipates heat and how well it handles the buildup of heat. The temperature grade applies to a properly inflated tire that is not overloaded. Underinflation, overloading or excessive speed can lead to more heat buildup. Excessive heat buildup can cause tires to wear out faster, or could even lead to tire failure. According to this NHTSA page, the Firestone Wilderness AT and Radial ATX II tires have a temperature rating of C.

Service Description

The service description consists of two things:

• Load Ratings: The load rating is a number that correlates to the maximum rated load for that tire. A higher number indicates that the tire has a higher load capacity. The rating "105," for example, corresponds to a load capacity of 2039 pounds (924.87 kg). A separate note on the tire indicates the load rating at a given inflation pressure.

• Speed Rating: The letter that follows the load rating indicates the maximum speed allowable for this tire (as long as the weight is at or below the rated load). For instance, S indicates that the tire can handle speeds up to 112 mph (180.246 kph). See the chart on this page for all the ratings.

 

Tire Traction

Did you know?

Safety grooving, the technique of cutting grooves into a paved road to increase tire traction, originated at a NASA research center. Learn about other NASA innovations in this interactive animation from Discovery Channel.

There are a lot of different terms used today in the tire industry. Some of them actually mean something and some do not. In this section, we'll try to explain what some of the terms mean.

All-Season Tires with Mud and Snow Designation

If a tire has MS, M+S, M/S or M&S on it, then it meets the Rubber Manufacturers Association (RMA) guidelines for a mud and snow tire. For a tire to receive the Mud and Snow designation, it must meet these geometric requirements (taken from the bulletin "RMA Snow Tire Definitions for Passenger and Light Truck (LT) Tires"):

1. New tire treads shall have multiple pockets or slots in at least one tread edge that meet the following dimensional requirements based on mold dimensions:

a. Extend toward the tread center at least 1/2 inch from the footprint edge, measured perpendicularly to the tread centerline.

b. A minimum cross-sectional width of 1/16 inch.

c. Edges of pockets or slots at angles between 35 and 90 degrees from the direction of travel.

2. The new tire tread contact surface void area will be a minimum of 25 percent based on mold dimensions.

The rough translation of this specification is that the tire must have a row of fairly big grooves that start at the edge of the tread and extend toward the center of the tire. Also, at least 25 percent of the surface area must be grooves.

The idea is to give the tread pattern enough void space so that it can bite through the snow and get traction. However, as you can see from the specification, there is no testing involved.

To address this shortcoming, the Rubber Manufacturers Association and the tire industry have agreed on a standard that does involve testing. The designation is called Severe Snow Use and has a specific icon (see image at right), which goes next to the M/S designation.

In order to meet this standard, tires must be tested using an American Society for Testing and Materials (ASTM) testing procedure described in "RMA Definition for Passenger and Light Truck Tires for use in Severe Snow Conditions":

Tires designed for use in severe snow conditions are recognized by manufacturers to attain a traction index equal to or greater than 110 compared to the ASTM E-1136 Standard Reference Test Tire when using the ASTM F-1805 snow traction test with equivalent percentage loads.

These tires, in addition to meeting the geometrical requirements for an M/S designation, are tested on snow using a standardized test procedure.They have to do better than the standard reference tire in order to meet the requirements for Severe Snow Use.

Categories: Car Repair, Automotive Repair

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